CSc 337 LECTURE 6: JAVASCRIPT Client-side scripting • client-side script: code runs in browser after page is sent back from server often this code manipulates the page or responds to user actions What is JavaScript? • a lightweight programming language ("scripting language") • used to make web pages interactive . insert dynamic text into HTML (ex: user name) . react to events (ex: page load user click) . get information about a user's computer (ex: browser type) . perform calculations on user's computer (ex: form validation) • a web standard (but not supported identically by all browsers) • NOT related to Java other than by name and some syntactic similarities JavaScript vs. Java • interpreted like Python, not compiled like Java • more relaxed syntax and rules . "looser" data types like Python + = JavaScript . variables don't need to be declared like Python . errors often silent (few exceptions) • key construct is the function rather than the class . "first-class" functions are used in many situations • contained within a web page and integrates with its HTML/CSS content Linking to a JavaScript file: script <script src="filename" type="text/javascript"></script> HTML <script src="example.js" type="text/javascript"></script> HTML • script tag should be placed in HTML page's head • script code is stored in a separate .js file • JS code can be placed directly in the HTML file's body or head (like CSS) • but this is bad style (should separate content, presentation, and behavior) A JavaScript statement: alert alert("message"); JS alert("IE6 detected. Suck-mode enabled."); JS output • a JS command that pops up a dialog box with a message Variables and types var name = expression; JS var age = 32; var weight = 127.4; var clientName = "Connie Client"; JS • variables are declared with the var keyword (case sensitive) • types are not specified, but JS does have types ("loosely typed") • Number, Boolean, String, Array, Object, Function, Null, Undefined • can find out a variable's type by calling typeof Number type var enrollment = 99; var medianGrade = 2.8; var credits = 5 + 4 + (2 * 3); JS • integers and real numbers are the same type (no int vs. double) • same operators: + - * / % ++ -- = += -= *= /= %= • similar precedence to Java • many operators auto-convert types: "2" * 3 is 6 String type var s = "Connie Client"; var fName = s.substring(0, s.indexOf(" ")); // "Connie" var len = s.length; // 13 var s2 = 'Melvin Merchant'; // can use "" or ' ' • methods: charAt, charCodeAt, fromCharCode, indexOf, lastIndexOf , replace, split, substring, toLowerCase, toUpperCase • charAt returns a one-letter String (there is no char type) • length property (not a method as in Java) • concatenation with + : 1 + 1 is 2, but "1" + 1 is "11" More about String • escape sequences behave as in Java: \' \" \& \n \t \\ • to convert between numbers and Strings: var count = 10; var s1 = "" + count; // "10" var s2 = count + " bananas, ah ah!"; // "10 bananas, ah ah!" var n1 = parseInt("42 is the answer"); // 42 var n2 = parseFloat("booyah"); // NaN • to access characters of a String, use [index] or charAt: var firstLetter = s[0]; var firstLetter = s.charAt(0); var lastLetter = s.charAt(s.length - 1); Comments (same as Java) // single-line comment /* multi-line comment */ JS • identical to Java's comment syntax • recall: 3 comment syntaxes • HTML:<!-- comment --> • CSS/JS:/* comment */ • Java/JS:// comment for loop (same as Java) for (initialization; condition; update) { statements; } JS var sum = 0; for (var i = 0; i < 100; i++) { sum = sum + i; } JS var s1 = "hello"; var s2 = ""; for (var i = 0; i < s.length; i++) { s2 += s1[i] + s1[i]; } // s2 stores "hheelllloo“ JS Math object var rand1to10 = Math.floor(Math.random() * 10 + 1); var three = Math.floor(Math.PI); JS • methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin, sqrt, tan • properties: E, PI Logical operators • Relational: > < >= <= • Logical: && || ! • Equality: == != === !== • most logical operators automatically convert types. These are all true: • 5 < "7" • 42 == 42.0 • "5.0" == 5 • The === and !== are strict equality tests; checks both type and value: • "5.0" === 5 is false Boolean type var iLikeJS = true; var ieIsGood = "IE6" > 0; // false if ("web dev is great") { /* true */ } if (0) { /* false */ } JS •any value can be used as a Boolean •"falsey" values: 0, 0.0, NaN, "", null, and undefined •"truthy" values: anything else •converting a value into a Boolean explicitly: •var boolValue = Boolean(otherValue); •var boolValue = !!(otherValue); Special values: null and undefined var ned = null; var benson = 9; var caroline; // at this point in the code, // ned is null // benson's 9 // caroline is undefined JS • undefined : has not been declared, does not exist • null : exists, but was specifically assigned an empty or null value • Why does JavaScript have both of these? if/else statement (same as Java) if (condition) { statements; } else if (condition) { statements; } else { statements; } JS • identical structure to Java's if/else statement • JavaScript allows almost anything as a condition while loops (same as Java) while (condition) { statements; } JS do { statements; } while (condition); JS • break and continue keywords also behave as in Java but do not use them in this class! Arrays var name = []; // empty array var name = [value, value, ..., value]; // pre-filled name[index] = value; // store element PHP var ducks = ["Huey", "Dewey", "Louie"]; var stooges = []; // stooges.length is 0 stooges[0] = "Larry"; // stooges.length is 1 stooges[1] = "Moe"; // stooges.length is 2 stooges[4] = "Curly"; // stooges.length is 5 stooges[4] = "Shemp"; // stooges.length is 5 PHP • two ways to initialize an array • length property (grows as needed when elements are added) Array methods var a = ["Stef", "Jason"]; // Stef, Jason a.push("Brian"); // Stef, Jason, Brian a.unshift("Kelly"); // Kelly, Stef, Jason, Brian a.pop(); // Kelly, Stef, Jason a.shift(); // Stef, Jason a.sort(); // Jason, Stef JS • array serves as many data structures: list, queue, stack, ... • methods: concat, join, pop, push, reverse, shift, slice, sort, splice, toS tring, unshift • push and pop add / remove from back • unshift and shift add / remove from front • shift and pop return the element that is removed Splitting strings: split and join var s = "the quick brown fox"; var a = s.split(" "); // ["the", "quick", "brown", "fox"] a.reverse(); // ["fox", "brown", "quick", "the"] s = a.join("!"); // "fox!brown!quick!the" JS • split breaks apart a string into an array using a delimiter • can also be used with regular expressions surrounded by /: var a = s.split(/[ \t]+/); • join merges an array into a single string, placing a delimiter between them Defining functions function name() { statement ; statement ; ... statement ; } JS function myFunction() { alert("Hello!"); alert("How are you?"); } JS • the above could be the contents of example.js linked to our HTML page • statements placed into functions can be evaluated in response to user events Event-driven programming • JS programs have no main; they respond to user actions called events • event-driven programming: writing programs driven by user events Event handlers <element attributes onclick="function();">... HTML <div onclick="myFunction();">Click me!</div> HTML Click me! HTML • JavaScript functions can be set as event handlers • when you interact with the element, the function will execute • onclick is just one of many event HTML attributes we'll use Buttons: <button> the canonical clickable UI control (inline) <button onclick="myFunction();">Click me!</button> HTML output • button's text appears inside tag; can also contain images • To make a responsive button or other UI control: 1. choose the control (e.g. button) and event (e.g. mouse click) of interest 2. write a JavaScript function to run when the event occurs 3. attach the function to the event on the control Accessing an element: document.getElementById var name = document.getElementById("id"); JS <img id="icon01" src="images/octopus.jpg" alt="an animal" /> <button onclick="changeImage();">Click me!</button> HTML function changeImage() { var octopusImage = document.getElementById("icon01"); octopusImage.src = "images/kitty.gif"; } JS output • document.getElementById returns the DOM object for an element with a given id <input> <!-- 'q' happens to be the name of Google's required parameter --> <input type="text" name="q" value="Colbert Report" /> <input type="submit" value="Booyah!" /> HTML output • input element is used to create many UI controls • an inline element that MUST be self-closed • name attribute specifies name of query parameter to pass to server • type can be button, checkbox, file, hidden, password, radio, reset, submit, text, ... • value attribute specifies control's initial text Text fields: <input> <input type="text" size="10" maxlength="8" /> NetID <br /> <input type="password" size="16" /> Password <input type="submit" value="Log In" /> HTML output • input attributes: disabled, maxlength, readonly, size, value • size attribute controls onscreen width of text field • maxlength limits how many characters user is able to type into field Text boxes: <textarea> a multi-line text input area (inline) <textarea rows="4" cols="20"> Type your comments here. </textarea> HTML output • initial text is placed inside textarea tag (optional) • required rows and cols attributes specify height/width in characters • optional readonly attribute means text cannot be modified DOM properties for form controls <input